Abstract: Since the 1990s the vaccine sector is experiencing a profound restructuring with the entrance of innovative biotech companies, developing country manufacturers, and wealthy private foundations. Since these developments and their consequences are rarely analysed in their interconnectedness, we argue, first, that a field approach focusing on the interrelations between the organisations in the vaccine sector is a fruitful way to unravel the complexities of the current changes. Second, a long-term historical-sociological analysis of this field is presented since the discovery of the smallpox vaccine around 1800. The current changes are interpreted as the third transformation of the field. After the shift from local to national vaccine fields, and then to an internationally coordinated field, the recent changes can be characterised as a shift to a more encompassing, diversified global field. These historical transformations can be explained by changing balances of power between the organisations with an interest in the field.